Electrolytic cell



P. FREEDLEY INVENTOR:

ORNEYS.'

ELECTROLYTIC CELL Filed Nov. 14, 1936 Jan. 14, 1941.

i 0 o o o! 0 Paul Frezdleg, BY

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Patented Jan. 14, 1941 UNITED STATES PATENT OFFICE 4 Claims.

This invention relates to electrolytic cells which are primarily adapted to the production of chlorine, a saline sodium hydroxide solution and hydrogen from a solution of sodium chloride. It will be apparent, however, that the subject of this invention may be applied to the electrolysis of solutions other than sodium chloride. More particularly, the present invention relates to a novel cathode for electrolytic cells.

In the field of electrolysis of alkali chlorides, one sort of cell heretofore used has been the socalled vertical, unsubmerged diaphragm type in which the cathode chamber is used to collect the caustic solution which percolates through the diaphragm, trickles down the surface of the cathode andis collected in a trap at the bottom of the cathode chamber. Consequently the cathode chamber is filled only with the hydrogen generated in the formation of the lye. This gas is removed and therefore there is no hydrostatic pressure to retard the percolation of the solution through the diaphragm, which usually consists of a relatively thin sheet of asbestos. The brine in the anode compartment percolates through the diaphragm and the cathode -by virtue of the hydrostatic pressure against the diaphragm. In this type of cell the sodium ion which is formed by the electrolytic decomposition of salt in the brine reacts with water, after discharge at the cathode, to produce sodium hydroxide and hydrogen. In order that there may be a minimum excess of water at the cathode, it is imperative that there be a relatively slow percolation of decomposing brine solution through the diaphragm to the cathode. The optimum rate of percolation through the diaphragm is one which will allow the maximum amount of salt to be decomposed by the current so that the resultant liquor will' have a high caustic content and a low salt content. When these conditions prevail the evaporation and salt removal costs for lye concentration and purification will be at a minimum. It is possible to regulate the rate of percolation by, selecting a diaphragm of the requisite thickness, but heretofore operators of electrolytic cells have failed to efliciently control the rate of percolation at different levels. Due to the difierences in hydrostatic pressure along the inner surface of the diaphragm the rate of percolation varies at diflerent levels and inefficiencies of operation result.

A further inefliciency in the operation of the electrolytic cells heretofore in use is caused by the fact that the path of the current through the cell is longer for the lower portions of'the cathode and anode than it is for the upper portions. Therefore a greater percentage of the impressed voltage is available for decomposition at the upper portion of the cell than in the lower 5 portion in the cells heretofore in use.

An additional loss is caused by the fact that the electrolyte flowing through the diaphragm carries with it a certain amount of chlorine and carbon dioxide which combine either with the 1 discharged sodium ions or with the sodium hydroxide which has been formed by the action of the sodium ions on water. This reaction reduces the amount of lye in the efliuent liquor and is thus responsible for a so-called reversion loss.

Therefore, the principal object of the present invention is to provide an electrolytic cell which will operate at a substantially uniform rate of percolation at difierent levels. A further object is to provide an electrolytic cell with increased current flow in the lower portion. A still further object is to reduce the reversion loss in electrolytic cells.

In the drawing,

Fig. I shows an axial section through an electrolytic cell.

Fig. II is a perspective view of a cathode constructed in accordance with the present invention.

Fig. III shows a modified construction of the cathode which is the subject of the present invention. 1

In Fig. I the anode chamber of an electrolytic cell is shown at l with an inlet 2 through which the solution to be electrolyzed may be introduced. Anodes 3 project into the anode chamber l which is separated from the cathode chamber 4 by the diaphragm 5 which is held against the inner surface of the perforated cathode 6. Three outlets for the reaction products are shown, comprising a sodium hydroxide solution outlet 1, a chlorine outlet '8, and a hydrogen outlet 9.

Fig. II shows cathode 6 in more detail with openings l0 varying with respect to size.

Fig. III shows a modified construction of cathode 6 with openings III varying with respect to their spacing.

In the constructions of both of Figs. II and III, the proportionate area of the electrode perforations ID as compared with the general extent or area of the electrode or cathode 6 diminishes downward progressively throughout the entire perforated area of the electrode, i. e., throughout the area over which the electrode extends between the level of the top row of perforations and the level of the bottom row. Reinforced by the perforated external cathode 6, the diaphragm .5 extending from top to bottom of the cell forms a permeable wall around the liquid electrolyte chamber I wherein the anodes 3 are exposed to contact with the electrolyte, which also comes in contact with. said cathode 6 through said diaphragm wall 5.

. Heretofore it has been generally thought that the rate of percolation was dependent almost entirely upon the thickness of the diaphragm. While it istrue that the thickness of the diaphragm affects the rate of percolation, it is not possible to effect an efiicient control of .the rate of percolation by a thickening of the diaphragm toward the lower portion of the cathode because to do so would raise the electrical resistance in the lower portion of the cell and .thus add to the inefficiencies with respect to unequal current distribution. I have found by virtue of a series of investigations that the free area of the diaphragm is equally as important as its thickness insofar as rate ofpercolation is concerned. I have also discovered a way to vary the free area of the diaphragm by varying the percentage of voids existing in the cathode by virtue of its perforations. With the free area of the diaphragm so controlled the rate of percolation is substantially uniform at different levels. This is accomplished by constructing the cathode in such a way that the percentage of voids at a given level is inversely proportional to the square root of 29k where g is the gravitational constant and h is the hydrostatic head at the level in question.

In addition to effecting a uniform rate of percolation, a cathode constructed according to this invention reduces the resistance in the lower part of the cell because the percentage of voids in the cathode is lower in the lower portion of the oathode. Therefore this cathode increases the flow of current by reducing the electrical resistance in the lower portion of the cell and hence gives greater conversion there. Moreover, by reducing the flow of liquor to a minimum, the amount of chlorine and carbon dioxide carried through to the cathode is at a minimum and therefore the reversion loss is kept at a minimum.

One advantage of the present invention is the increased efliciency made possible by a controlled, uniform flow at different levels of the cathode. A further advantage is the regulation of the electrical resistance of the cell so as to eflect uniform distribution of current. A still further advantage is the increase of cell capacity made possible by the fact that the hydrostatic head may be increased in view of the control over the rate of percolation.

Another advantage is that the control of the rateof percolation makes it possible to obtain a solution containing a greater proportion of sodium hydroxide and a lesser proportion of salt.

Consequently great savings in cost of concentration and cleaning are effected. A final advantage is a reduction in reversion losses.

It will be understood that the foregoing description refers to but one embodiment of the present invention and that while the description has particular reference to the electrolysis of a brine solution, the present invention may be applied to the electrolysis of other solutions. Furthermore, it is to be understood that various modifications of cathode construction may come within the scope of the present invention. Thus the percentage of voids in the cathode may be fixed by varying the number of holes or slots at different levels or by varying their size, and this may be accomplished by punching out parts of the cathode material or by constructing the cathode in the fashion of a screen or the like. Moreover, it is to be understood that the percentage of voids in the cathode may be regulated by varying the perforations in a relatively few steps.

Having thus described my invention, I claim:

1. In an electrolytic cell of the type comprising a diaphragm that is unsubmerged at one side,

the combination with a liquid electrolyte chamber having a permeable wall extending between different levels of the cell, and an electrode in said chamber exposed to contact with the electrolyte therein, of a coacting electrode external to said permeable chamber wall and in contact therewith, and exposed therethrough to contact with said electrolyte, and perforated and extending over an area between different cell levels, the proportionate area of the electrode perforations as compared with the area of the electrode diminishing progressively downward throughout the entire perforated area of the electrode, from the top of said perforated area to the bottom thereof, whereby the rate of percolation is substantially uniform over the perforated area of the electrode.

2. The apparatus as set forth in claim 1, wherein the-electrode external to the permeable chamber wall is a cathode, and its perforations in number from the top of the perfo rated area of the cathode downward to the bottom of said area.

3. The apparatus as set forth in claim 1, wherein the electrode external to the permeable chamber wall is a cathode, and its perforations diminish in size from the top of the perforated area of the cathode downward to the bottom of said area.

4. The apparatus as set forth in claim 1, wherein the electrode external to the permeable chamber wall is a cathode substantially circular in cross section, and the perforation thereof is substantially uniform around the circumference of the cathode.

PAUL FREEDLEY. 

